Occasionally, a pilot may forget to reset the barometric pressure reference setting to standard (29.92″ Hg) when transitioning through the transition altitude. This can lead to the aircraft flying at a different altitude than that assigned. Aircraft cruise within flight routes at assigned altitudes. If an aircraft is not flying at its assigned altitude, it becomes a threat to other aircraft operating in the same flight route.
Also, transition altitude varies from area to area and it can be difficult to remember the transition altitude value. Therefore, there exists a need to alert a flight crew when the barometric pressure reference setting has not been adjusted after transitioning through a transition altitude.